CHANGES IN A MUSCLE DURING CONTRACTION. 81 



bulging, a thickening and shortening, over a greater or less part of the 

 length of the fibre, has been fixed by the osmic acid or other reagent. Such 

 a bulging obviously differs from a normal contraction in being confined to a 

 part of the length of the fibre, whereas, as we have said, a normal wave of 

 contraction, being very much longer than any fibre, occupies the whole 

 length of the fibre at once. We may however regard this bulging as a very 

 short, a very abbreviated wave of contraction, and assume that the changes 

 visible in such a short bulging also take place in a normal contraction. 



Admitting this assumption, we learn from such preparations that in the 

 contracting region of the fibre, while both dim and bright bands become 

 broader across the fibre, and correspondingly thinner along the length of the 

 fibre, a remarkable change takes place between the dim bands, bright bands, 

 and granular lines. We have seen that in the fibre at rest the intermediate 

 line in the bright band is in most cases inconspicuous ; in the contracting 

 fibre, on the contrary, a dark line in the middle of the bright band in the 

 position of the intermediate line becomes very distinct. As we pass along 

 the fibre from the beginning of the contraction wave to the summit of the 

 wave, where the thickening is greatest, this line becomes more and more 

 striking, until at the height of the contraction it becomes a very marked 

 dark line or thin dark band. Pari passu with this change, the distinction 

 between the dim and the bright bands become less and less marked ; these 

 appear to become confused together, until at the height of the contraction, 

 the whole space between each two now conspicuous dark lines is occupied by 

 a substance which can be called neither dim nor bright, but which in con- 

 trast to the dark line appears more or less bright and transparent. So that 

 in the contracting part there is, at the height of the contraction, a reversal 

 of the state of things proper to the part at rest. The place occupied by the 

 bright band, in the state of rest, is now largely filled by a conspicuous 'dark 

 line which previously was represented by the inconspicuous intermediate 

 line, and the place occupied by the conspicuous dim band of the fibre at rest 

 now seems by comparison with the dark line the brighter part of the fibre. 

 The contracting fibre is, like the fibre at rest, striated, but its striation is dif- 

 ferent in its nature from the natural striation of the resting fibre ; and it is 

 held by some that in the earlier phases of the contraction, while the old nat- 

 ural striation is being replaced by the new 1 striation, there is a stage in which 

 all striation is lost. 



We may add that the outline of the sarcolemma, which in the fibre at 

 rest is quite even, becomes during the contraction indented opposite the 

 intermediate line, and bulges out in the interval between each two interme- 

 diate lines, the bulging and indentation becoming more marked the greater 

 the contraction. 



56. We can learn something further about this remarkable change by 

 examining the fibre under polarized light. 



When ordinary light is sent through a Nicol prism (which is a rhomb of Ice- 

 land spar divided into two in a certain direction, the halves being subsequently 

 cemented together in a special way) it undergoes a change in passing through the 

 prism arid is said to be polarized. One effect of this polarization is that a ray 

 of light which has passed through one Nicol prism will or will not pass through a 

 second Nicol according to the relative position of the two prisms. Thus, if the 

 second Nicol be so placed that what is called its "optic axis " be in a line with or 

 parallel to the optic axis of the first Nicol the light passing through the first Nicol 

 will also^pass through the second. But if the second Nicol be rotated until its 

 optic axis is at right angles with the optic axis of the first Nicol none of the light- 

 passing through the former will pass through the latter; the prisms in this position 

 are said to be "crossed." In intermediate positions more or less light passes 

 through the second Nicol according to the angle between the two optic axes. 



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